The invention relates to a method for coating the inside of a tube of an electrically insulating material by a second electrically insulating material by reactive deposition of the second electrically insulating material from a gas mixture which is passed through the tube, the deposition being activated by a plasma which is reciprocated in the tube and is produced by microwaves.
The invention furthermore relates to a device for internally coating a tube of an electrically insulating material by a second electrically insulating material by reactive deposition of the second electrically insulating material from a gas mixture which is passed through the tube, the device comprising a gas supply system for supplying the gas mixture to the tube, a microwave generator and a cylindrical microwave resonator for producing and maintaining a plasma in the gas mixture in the tube, means to reciprocate the plasma in the longitudinal direction of the tube and means for heating the tube.
A method and device of this type are known inter alia from Appl. Phys. Lett. 28 (1976) 645-646 and Topics in Current Chemistry 89 (1980) 107-131. These literature references describe the manufacture of optical fibres according to the PCVD method. In this method, light-conducting material is deposited as the second electrically insulating material by means of the plasma on the inside of the tube from the gas mixture which comprises volatile starting materials. The tube consists either of synthetically produced amorphous silica or of amorphous silica manufactured from quartz crystals by melting (fused silica, quartz glass), which may optionally be doped. The tube may optionally consist both of synthetically manufactured and of amorphous silica (fused silica, quartz glass) manufactured from quartz crystals by melting and which silica may optionally be doped. After a quantity of light-conducting material corresponding to the optical fibre construction to be obtained has been deposited, the tube is made to collapse to form a solid preform from which optical fibres are drawn.
As is described in greater detail in the above literature references, temperatures of the wall of the tube between 1100.degree. and 1300.degree. C. are required for the deposition of adhering layers of light-conducting material. In order to ensure these temperatures during the coating step, furnaces are used as additional energy sources. Furnaces which are moved over the tube to be coated synchronously with the microwave resonator, or a stationary furnace in which the microwave resonator can be moved over the tube to be coated inside the furnace can be used for this purpose.
In both cases the microwave resonator in which the plasma is produced is reciprocated along a fused silica tube. Due to the comparatively low temperature, the deposition of the doped fused silica layers take place only in the range of the plasma, any thermally activated reaction if occurring at all, is of little practical importance. The two modes of operation differ as to how the required deposition temperature of preferably approximately 1200.degree. to 1260.degree. C. is maintained. In one case two furnaces which are flange-mounted to the resonator move over the tube together with said resonator. Of course the tubes should have such a length that the tube furnace, even in the extreme positions, do not inadmissibly heat the means for clamping the tube because these means are vacuum-tight and have to remain vacuum-tight. Therefore the fused silica tube should be at least three times as long as the actual deposition zone. This requires much space and causes extra costs.
In the second embodiment a stationary furnace is used in which an optionally cooled resonator moves. Although in this case the deposition length is only negligibly shorter than the tube length, the stationary furnace must have a considerable inner volume and consequently is bulky and expensive. Its electric connection power is, for example, more than 20 kW. The length of the resonator corresponds approximately to half a vacuum wavelength. In this microwave resonator a slightly modified coaxial mode is used in which the electrical field lines extend radially.
It has been found that in this resonator microwave energy leaks away through the axial bores. This is disadvantageous because screening measures have to be taken to keep below admissible limits the microwave energy which can be detected outside the device.
A further disadvantage of this resonator is caused by its dimensions. Although the TEM basic mode should be excited, higher modes are already possible with the given dimensions, for example (the TE11-Coaxial Conductor Mode) which upon excitation produce a deviation from the rotational symmetry of the field distribution. When such modes are to be avoided, because for example an asymmetric deposition, should be avoided, a fixed diameter D (=Di+Da/2; D.sub.i and D.sub.a meaning inner and outer diameter respectively) of the resonator should satisfy the following condition: EQU D.ltoreq..lambda./.pi.
wherein .lambda. (=vacuum wavelength)=12.24 cm at 2.45 GHz, and .lambda./.pi.=3.9. At, D.sub.i =7.1 cm and D.sub.a =3.1 cm, D is already 5.1 cm. An increase in the diameter of the resonator, for example for larger tube diameters, is therefore risky. Moreover the stationary furnace then should also be still larger than it is already now.